The present invention relates to a nonwoven composite structure having at least two melt-extruded nonwoven layers. More particularly, the present invention relates to a nonwoven composite structure having at least two melt-extruded layers in which the fibers of at least one layer are prepared by melt extrusion of a mixture of an additive and a thermoplastic polymer, which additive imparts to the surfaces of said fibers, as a consequence of the preferential migration of said additive to the surfaces of said fibers as they are formed, at least one characteristic which is different from the surface characteristics of fibers prepared from said thermoplastic polymer alone.
Multilayer nonwoven composites and laminates are not new, although the earlier structures differ significantly from those provided by the present invention. By way of illustration, some of the earlier structures are described in the paragraphs which follow.
U.S. Pat. No. 3,770,562 to Newman discloses a composite nonwoven fabric. A soft and flexible spunbonded web is impregnated with a soft thermoplastic binder, dried, then plied by heat and pressure to at least one substantially binder-free fibrous layer. Such fibrous layer typically is a sheet of cellulose tissue, rayon, or cotton card or garnett web, or the like.
U.S. Pat. Nos. 3,973,068 and 4,070,218 to Weber describe a soft, nonwoven web which is produced by adding directly to a thermoplastic polymer at the time of extrusion a lubricating agent having and HLB number in the range of 8 to 20 and a molecular weight in the range of from 200 to 4,000. The lubricating agent is uniformly distributed into the polymer as extruded into filaments. The lubricating agent is forced to the surfaces of the fibers by subjecting the formed web to a stringent heat treatment.
European Patent Application No. 87307470.2, Publication No. 0 260 011 A2, describes fluorochemical oxazolidinones. The compounds are stated to be particularly useful as additives in synthetic organic polymer melts which, when melt-extruded, give fibers and films having low surface energies, oil and water repellency, and resistance to soiling. Suitable polymers include polyamides, polyesters, and polyolefins. ZONYL.RTM. fluorochemicals recently were advertised in the Jul. 16, 1990 issue of Chemical and Engineering News as additives for polymers which migrate to the surface and orient themselves with their perfluoroalkyl groups facing outward. Other references relating to the incorporation of fluorochemicals into such polymers as polyolefins include, among others, D. R. Thompson et al., "New Fluorochemicals for Protective Clothing," INDA-TEC 90 Conference, Jun. 5-8, 1990 and U.S. Pat. Nos. 4,855,360 to Duchesne et al. and 4,863,983 to Johnson et al.
Low-temperature-spinnable polypropylene compositions for the preparation of spunbonded nonwovens are described in published Japanese Patent Application No. 61,155,437 [Chem. Abstr., 105:192786s (1986). The compositions contain a crystal-nucleating agent, e.g., 0.065 percent bis(1-tert-butylperoxy-1-methylethyl)benzene and 0.2 percent dibenzylidenesorbitol.
U.S. Pat. No. 3,969,313 to Aishima et al. describes thermoplastic composite compositions comprising a thermoplastic material, a polyolefin, and a reactive inorganic filler. The filler is prepared by the reaction of a metal carbonate, hydroxide, or oxide with a defined unsaturated aliphatic or aromatic carboxylic acid. See also U.S. Pat. No. 3,926,873, also to Aishima et al., which describes a similar composition lacking the polyolefin component.
A method of making a structural member from a prepeg sheet of fusible resin microfibers and heat-resistant reinforcing fibers is described in U.S. Pat. No. 4,104,340 to Ward. Alternating layers of fusible resin microfibers and heat-resistant reinforcing fibers such as glass fibers are formed on a moving forming wire. The resulting multi-layered sheet then is molded under heat and pressure to form the structural member.
U.S. Pat. No. 4,196,245 to Kitson et al. discloses a composite nonwoven fabric comprising adjacent microfine fibers in layers. The fabric comprises at least two hydrophobic plies of microfine fibers and at least one nonwoven cover ply which may be an apertured film, a spunbonded ply, or an air-laid, wet-laid, or carded ply of fibers. The hydrophobic plies typically are prepared by meltblowing.
A disposable absorbent nonwoven structure is described in U.S. Pat. No. 4,287,251 to King and Boyd. The structure comprises alternate layers of absorbent nonwoven material and nonwoven hydrophobic thermoplastic material, minimally bonded together. The absorbent layers may comprise spunbonded rayon webs or webs or air-laid, wet-laid, or carded rayon fibers of staple length.
U.S. Pat. No. 4,375,718 to Wadsworth and Hersh describes a method of making an electrostatically charged filtration medium. Briefly, a web made of nonconductive thermoplastic fibers is contacted on each side with a more conductive web to form a combined web. The combined web is charged with electrically charged particles from corona charging elements on opposite sides of the web. The nonconductive web typically is made from polypropylene fibers.
A nonwoven fabric and a method for its production is disclosed in U.S. Pat. No. 4,377,615 to Suzuki and Igaue. The fabric comprises an upper layer having a substantially smooth surface and a lower layer having a density lower than that of the upper layer. Both layers are fibrous and contain adhesive bonding materials.
U.S. Pat. No. 4,436,780 to Hotchkiss et al. describes a nonwoven wiper laminate. The laminate consists of a relatively high basis weight middle layer of meltblown thermoplastic microfibers and, on either side, a lightweight layer of generally continuous filament thermoplastic fibers having a larger fiber diameter. A preferred laminate consists of a polypropylene meltblown layer having on either side a polypropylene spunbonded layer. A similar material is described in U.S. Pat. No. 4,906,513 to Kebbell and Watts, in which the center meltblown layer has mixed therein other fibers or particles. See also U.S. Pat. No. 4,374,888 to Bornslaeger which describes a similar laminate for a recreation fabric, in which the outer layers are treated for resistance to ultraviolet radiation and/or flame retardancy. A two-layered meltblown-spunbonded fabric is disclosed in U.S. Pat. No. 4,041,203 to Brock and Meitner, and a three-layered fabric comprising a carded web sandwiched between two spunbonded layers is described in U.S. Pat. No. 4,039,711 to Newman.
U.S. Pat. No. 4,508,113 to Malaney describes a microfine fiber laminate. A preferred embodiment comprises a three-ply hydrophobic microfine fiber structure sandwiched between and fuse-bonded to two layers of conjugate fibers having a low melting sheath and a high melting core. The inner ply of the three-ply structure is relatively high melting while the two outer plies are low melting.
An extensible microfine fiber laminate is disclosed in U.S. Pat. No. 4,555,811 to Shimalla. A preferred embodiment comprises an inner creped hydrophobic microfine fiber structure sandwiched between and bonded to two reinforcing layers of nonwoven fibers. The inner structure may comprise two or more plies bonded together, which plies preferably are prepared by meltblowing a thermoplastic polymer. The reinforcing layers preferably consist of spunbonded webs made up of sheath/core bicomponent fibers.
U.S. Pat. No. 4,604,313 to McFarland and Lang relates to the selective layering of superabsorbents in meltblown substrates. A first layer of meltblown fibers containing wood fibers is formed on a continuous foraminous belt. A second layer is formed on the first layer, the second layer containing both wood fibers and a superabsorbent and being integrally connected to the first layer. See also U.S. Pat. Nos. 4,655,757 and 4,724,114, both also to McFarland and Lang.
U.S. Pat. No. 4,610,915 to Crenshaw et al. relates to a two-ply nonwoven fabric laminate. The first ply is a synthetic fibrous nonwoven material and the second ply is a fibrous nonwoven material. The two plies are bonded together by means of a flexible, soft latex binder which penetrates each ply to a depth of from about 20% to about 80% of its thickness. The first ply typically is a spunbonded web made from a thermoplastic polymer, the most common of which are rayon, polyester, polypropylene, and nylon. The second ply can be an air-laid or a wet-laid cellulosic pulp sheet, with a tissue sheet being preferred. See also U.S. Pat. No. 4,588,457 to Crenshaw et al.
A microporous multilayer nonwoven material for medical applications is described in U.S. Pat. No. 4,618,524 to Groitzsch and Fahrbach. The material consists of a layer of microfibers covered on opposite sides with nonwoven layers, all of the layers being bonded together with a pattern of water repellent and, preferably, elastic paste members sufficiently penetrating through the layers.
A multilayer nonwoven fabric is disclosed in U.S. Pat. No. 4,668,566 to Braun. Such fabric comprises at least two layers of nonwoven web adjacent and bonded to each other. One of the layers is composed of polypropylene fibers and another of the layers is composed of polyethylene fibers.
U.S. Pat. No. 4,714,647 to Shipp, Jr. and Vogt describes a meltblown material with a depth fiber size gradient. The material is useful as a filter medium and is formed by sequentially depositing layers of meltblown thermoplastic fibers, having the same composition but different sizes, onto a collector. See also U.S. Pat. No. 4,904,521 to Johnson et al. which describes multi-layered nonwoven wiper having a number of interbonded meltblown layers, in which the inner layers have smaller average pore sizes for liquid-holding capacity.
U.S. Pat. No. 4,753,843 to Cook and Cunningham describes an absorbent, protective nonwoven fabric. The fabric has one or more center layers of meltblown polypropylene microfibers sandwiched between one or more meltblown surface layers. The surface layers are composed of meltblown polypropylene microfibers which have been rendered hydrophilic by spraying the fibers as they are formed with an aqueous solution of a nonionic surfactant.
A laminated fibrous web is described in U.S. Pat. No. 4,761,322 to Raley. The web comprises a first fibrous layer and a second fibrous layer, in which the second fibrous layer is bonded to and of lower density than the first fibrous layer. The fibers in the second fibrous layer are less bonded to one another than fibers in the first fibrous layer are bonded to one another. In addition, the first and second fibrous layers are less bonded to each other than fibers in the first fibrous layer are bonded to one another. Preferably, the fibrous layers are spunbonded nonwoven webs prepared from thermoplastic polymers, such as polypropylene, polyethylene, polyesters, polyamides, and polyurethanes. The bonding differences result from and are controlled by thermal pattern-bonding.
U.S. Pat. No. 4,766,029 to Brock et al. discloses a semi-permeable nonwoven laminate useful as a house wrap. The laminate consists of three layers. The two exterior layers are spunbonded polypropylene and the interior layer is a two-component meltblown layer of polyethylene and polypropylene. The laminate is calendared after formation.
A fabric for protective garments is described in U.S. Pat. No. 4,772,510 to McClure. The fabric comprises an outer polymeric film of poly(vinyl fluoride), poly(vinylidene fluoride), or copolymers thereof, bonded to a second film of poly(vinyl alcohol) polymer or copolymer which in turn is bonded to a textile fabric which preferably is a nonwoven fabric. Bonding of the various layers is accomplished by known techniques.
A multilayer nonwoven fabric is disclosed in U.S. Pat. No. 4,778,460 to Braun et al. The fabric comprises at least two layers of a nonwoven web, the fibers of at least one web having a bilobal cross-section. In a preferred embodiment, the fabric consists of two layers, with the fibers of the first web having a bilobal cross-section and the fibers of the second web having a trilobal or branched cross-section. In another preferred embodiment, the second layer is rendered wettable by either incorporating a wetting agent in the polymer before melt-extruding or applying a solution of a wetting agent to the nonwoven web after it is formed. The fibers can be prepared from a variety of polymers, with polyolefins being preferred. Suitable melt-extrusion processes include spunbonding and meltblowing, with spunbonding being preferred. The layers typically are stabilized by thermal bonding in discrete, compacted areas.
U.S. Pat. No. 4,784,892 to Storey and Maddern discloses a laminated nonwoven material which comprises a layer of a coformed nonwoven material, i.e., meltblown polymeric microfibers intermixed with wood pulp fibers, cellulose fibers, or absorbent or superabsorbent particles, and a layer of meltblown polymeric microfibers which also may be a coformed nonwoven material as already described. Preferably, the laminate comprises a coformed material sandwiched between to meltblown layers. The layers are bonded together, such as by ultrasonic energy or heated calendaring rolls. Suitable polymers include polyethylene, polypropylene, polyester, and nylon, although polypropylene is preferred.
U.S. Pat. No. 4,818,585 to Shipp, Jr. describes an agricultural protective fabric which comprises at least two layers. The first layer is a spunbonded nonwoven web prepared from a polymer which is resistant to being degraded by the environment during the growing season. The second layer is a meltblown nonwoven web prepared from a polymer which degrades during the growing season. The first layer typically is prepared from polypropylene treated with ultraviolet light stabilizers; "treatment" apparently means incorporation into the polymer before melt-processing of an ultraviolet light stabilizing additive. The second layer is prepared from polypropylene which has not been so treated. Other polymers can be used for either or both layers and include polyethylene, polyester, nylon, and the like.
A health-care laminate is described in U.S. Pat. No. 4,818,597 to DaPonte et al. The laminate comprises five layers including a central meltblown nonwoven layer made from a polar polymer such as poly(ethylene-vinyl acetate). Insulative layers are disposed on both sides of the central layer. The insulative layers are nonwoven meltblown webs which are formed of the same or different nonpolar, heat-resistant thermoplastic polymers, such as polypropylene and polyethylene. Finally, outer layers on either side of the insulative layers are formed of nonwoven spunbonded webs. The outer layers may be prepared from the same or different thermoplastic polymer. Suitable polymers include polypropylene, polyethylene, ethylene-propylene copolymers, and polyethylene-polypropylene blends. The layers are calendared and embossed.
International Application No. PCT/GB87/00211, having Publication No. WO 87/05952, filed in the names of Maddern and Currie, describes a multilayer nonwoven fabric comprising at least one spunbonded layer which has been impregnated with a thermal stabilizing agent before hot calendaring one at least one side. The thermal stabilizing agent preferably is a fluorocarbon. The stabilizing agent is stated to form an antistatic and fluid-repellant coating and to resist surface fuzzing of the fabric.
Finally, U.S. Pat. No. 3,738,884 to Soehngen describes a nonwoven mat or fabric composed of partially overlapping regions aligned in the machine direction. While not a true multilayered structure as the term is used herein, it is noted at this point for the sake of completeness. The mat or fabric is produced by having several nozzles aligned generally in the cross direction. The spray patterns may overlap to the extent of intermingling filaments during the formation of the mat or fabric. Different materials may be sprayed simultaneously from the nozzles to give a fabric characterized by the presence of longitudinally extending portions or stripes having different visual or structural properties.
Notwithstanding the wide variety of multilayered structures already known, there still is a need for a multilayered structure composed of multiple nonwoven layers, in which the surface characteristics of the fibers making up the layers are determined at will at the time of the formation of the layers, without the need for any external or postformation treatment of any kind.